After the newest patch has been released I took the jump and bought the game. I started a new game and chose war plans in a way that Italy joined on the side of the central powers. On turn 2 Italy takes Nice - the Entente plays an event that some North African troops are deployed on the border to Italy et viola - now I have an Italian army AND a French corps in Nice. They co-exist peacefully and won't disappear. I think there is a bug in the event logic not checking if the provinces for the new forces are still under French control.

Was not able to replicate your problem but have not seen this one before; sometimes events conflict with each other and produce unexpected results. WW1G will do odd thing at times but your issue does not appear to be a graphics anomaly so once you are more familiar with the game you can edit the save files to correct these sorts of errors.

Believe it or not, they are exceptional but have seen other event errors when Italy is a Central Power.

If you are not comfortable editing game files (after saving the original someplace safe), replaying the turn might be a cure although this may be a bit drastic particularly if all else went well last time.